Molecular sieves of the crystalline zeolite type are well known in the art and now comprise over 250 species of both naturally occurring and synthetic compositions. In general, the crystalline zeolites are aluminosilicates whose frameworks are formed from AlO.sub.4 and SiO.sub.4 tetrahedral joined by the oxygen atoms and characterized by having pore openings of uniform dimensions, having significant ion-exchange capacity and being capable of reversibly desorbing an adsorbed phase which is dispersed through the internal voids of the crystal, without displacing any atoms which make up the permanent crystal structure.
In the prior knowledge, the method of preparation of Zeolite-A phase is generally known from sodium silicate/aluminate and from kaolinite. The smallest building units of Zeolite-A are tetrahedral in which a central metal ion like Sl.sup.4+ or Al.sup.3+ is tetrahedrally surrounded by four oxygen atoms. This way, the sodalite unit is formed which is composed of 24 tetrahedra. If sodalite units are linked together at their six quadratic planes via quadratic prisms, the structure of Zeolite-A is formed. It contains large cages, with a near spherical shape and free diameter of 1.14 nm. Each of this cages is connected with six neighboring cages via-8 membered rings (8-MR) or windows with a crystallagraphic diameter of 0.41 nm. Actually, the effective diameter and hence, the critical pore width of Zeolite-A can be manipulated by proper choice of the nature of the cations which are required to compensate the negative lattice charge brought about by each AlO4.sup.2- tetrahedron. The effective pore width of zeolite Na-A is 0.4 nm. The general chemical formula of the synthetically produced, anhydrous, porous, Zeolite-A expressed in terms of moles may be as follows: 1.0.+-.0.2 Na.sub.2 O:Al.sub.3 O.sub.2 :nSiO.sub.2 wherein n has a value from 1 to 1.5 These are commercially more useful as adjuvants or detergent builders as they have proven high exchange capacity.
The detergents used in the household, in commercial establishments and in industry, frequently contain large quantities of condensed phosphates, particularly tripolyphosphates. These are provided to sequester the hardness formers of water and are responsible, to a great extent, for increasing the cleaning power of the capillary active washing substances. The phosphate content of these agents leads to environmental problem viz. eutrophication and hence restrictions on these have been imposed in many countries, throughout the world. A solution to this is set forth with the development of a process for synthesis of economically viable material (FAZ-A) which can be a potential substitute for phosphates in detergents.
Use of flyash as a raw material for production of zeolites would provide an inexpensive alternate to commercially available zeolites (as the process involves replacement of conventional alumina and silica sources with flyash) with concomitant resolution of flyash disposal problems. Special emphasis has been given to synthesis of detergent grade Zeolite-4A from flyash as this particular zeolite is the most suitable substitute for conventional phosphatic detergent builder. Although extensive literature on synthetic methods of Zeolites (Kessler, H, 9th Proc. Inst. Zeo Cong. 1,-73-125, 1992 Dwyer, J. Chem Ind., 7,258-269, 1984 Bhattacharya, A. Das, J., Mitra, S. and Ray, S. K. J. Chem Tech Biotech., 54, 399-407, 1992) using salts of silicates, phosphates/aluminates, clay and clay minerals are available; use of waste materials to synthesize zeolites have not received much attention. Hydrothermal synthesis of kaolinite from blast furnace slag has been reported (Miyawaki, Ritsuo, Tomwa, Shimji, Inukai, Kogaku, 34(1)11-12, 1994 (Japanese). - Harvath,l Gyulo and Sasvari Judit, Kahazt, 127(2), 78-81, 1994) which is a precursor for zeolite synthesis. Formation of sodiumaluminate-silicate hydrates in the Bayer's liquor has been reported recently (Harvath, Gyulo and Sasvari Judit, Kahazi, 127(2), 78-81, 1994), while synthesis of zeolite form waste water treatment sludges and incineration ashes have also been reproted (Sawazaki, Tesuo, Jpn, Kokai Tokkya, JP 06,239, 612 Aug. 30, 1994 Sawazaki, Tetsuo. Jpon Kokai Tokkya Kohio, JP 06, 256-012 Sep. 12, 1994)
In particular, use of flyash to synthesize zeloltes have been attempted by a very few researchers. Pioneering research in the field of zeolite detergency has been initiated by Schwuger et al. Zeolites have been used a phosphate substitute in laundry detergents on a large scale in USA, Japan and Germany. For example, Procter and Gamble Co. marketed "Tide", a heavy duty powder detergent in USA, while leading detergent "Top" marketed by Lion corporation was reformulated to phosphate-free zeolite built product in Japan. In Germany, Henkel KGaA has introduced zeolite based detergent brand "Prodixan" while "Henko" a new product based on zeolite formulation has recently been marked in India.
It is also believed that leading companies like Procter and Gamble and Hindustan Lever are already using zeolites in some of their formulation in India. Recently, work has been published by Bhat emphasizing the use of zeolite as a versatile detergent builder.
In general, zeolite synthetics from flyash involves alkaline treatment using caustic soda solution (NaOH) at temperature of 80-100.degree. C. Most previous studies (Singer, Arich, Berkagat, Vadium, Env. Sci & Technol 29(9), 1748-53, 1995 Cheng-Fang, Lin and Hsing-chang His, Env., Sci & Technol 29, 1109-1117, 1995) evaluated conversion of flyash to zeolite like materials under ambient pressure conditions and did not properly explore the factors affecting zeolite formation. The direct treatment of flyash using caustic soda (NaOH) solution requires prolonged hydrothermal treatment for 24-48 hrs and is an energy intensive step. This is a major disadvantage and has been overcome in the invention by using optimised fusion step. The direct treatment of flyash with caustic soda (NaCH) solution also favours the formation of sodalite which is an undesirable component for detergent formulations. In addition, it has high fraction of unreacted flyash and the yield of zeolite phases is significantly low. In particular, Zeolite. A formation is negligible without modification of SiO.sub.2 /Al.sub.2 O.sub.3 ratio in the reaction mixture by way of alumina addition.